1. Field of the Invention
The present invention relates to an electrical load driving apparatus for supplying a current to an electrical load through a plurality of transistors.
2. Description of Related Art
Japanese Patent Application Laid-open number 2003-158447 discloses such an electrical load driving apparatus having a structure in which a plurality of power supply wires are connected in parallel, one connection node of the power supply wires being connected to one end of an electrical load which is connected to one of the high voltage side and the low voltage side of a power source, the other node being connected to the other of the high voltage side and the low voltage side of the power source, and each of the power supply wires including a current supply transistor interposed therein. Accordingly, in this structure, a plurality of the current supply transistors are connected in parallel with one another, and connected in series to the electrical load.
In this apparatus, the current supply transistors are turned on during a current supply period in which the electrical load should be continuously supplied with a current.
According to the above structure, it is possible to maintain supply of a current to the electrical load even if one of the current supply wires is broken.
Further, the above electrical load driving apparatus is configured to generate sole-on periods within the current supply period, only a corresponding one of the current supply transistors being turned on during each sole-on period, and to monitor the voltage at the output terminals of the current supply transistors connected in parallel with one another during the sole-on period to determine whether or not the current supply transistor to be turned on within the sole-on period has an open failure in which the transistor is fixed to the off state.
This configuration makes it possible to detect an open failure of each current supply transistor, and also to detect a wire breakage of each current supply wire as an open failure of the current supply transistor regardless whether the wire breakage occurs at a portion on the near side of the current supply transistor, or at a portion on the far side of the current supply transistor with respect to the electrical load.
However, the above conventional electrical load driving apparatus has a disadvantage in reliability, because the sole-on period is generated within the current supply period.
More specifically, in a case where the electrical load is not a load that should be driven only a short period in response to a certain event, but a load that should be drive continuously (for example a relay for relaying power to an ignition system of a vehicle), since the failure detection is performed by turning on one of the current supply transistors during each sole-on period all the while the vehicle is running, the current supply transistors may degrade quickly.
In addition, if one of the current supply transistors has an open failure or if one of the current supply wires is broken, current supply to the electrical load is interrupted during each corresponding sole-on period.